Bidirectional switching assisted by interlayer exchange coupling in asymmetric magnetic tunnel junctions

Abstract

We study the combined effects of spin transfer torque, voltage modulation of interlayer exchange coupling and magnetic anisotropy on the switching behavior of perpendicular magnetic tunnel junctions (p-MTJs). In asymmetric p-MTJs, a linear-in-voltage dependence of interlayer exchange coupling enables the effective perpendicular anisotropy barrier to be lowered for both voltage polarities. This mechanism is shown to reduce the critical switching current and effective activation energy. Finally, we analyze the possibility of having switching via interlayer exchange coupling only.

FIG. 1:

(Color online) (a) MTJ band diagram. The parameter $\delta ={ϵ}_L^{\uparrow (\downarrow )}-{ϵ}_R^{\uparrow (\downarrow )}$ controls the asymmetry of the MTJ.The bottom of the spin up (down) bands in the single orbital tight-binding approach is ${ϵ}^{\uparrow (\downarrow )}-6t$, where t is the nearest neighbor hopping parameter. The inset shows a sketch of an asymmetric p-MTJ with m and m_p corresponding to the unit vectors in the direction of the magnetization of the free and fixed layer, respectively. Panels (b) and (c) show the voltage dependence of spin transfer torque and non-equilibrium interlayer exchange coupling, respectively, for different MTJ asymmetries δ, as defined in (a).

FIG. 2:

(Color online) Critical current density as function of the asymmetry parameter δ. The red circles (blue triangles) show the trend for P to AP (AP to P) switching. (b) Normalized effective activation energy as function of applied voltage for different p-MTJ asymmetries for P to AP (V > 0) and AP to P (V < 0) switching.

FIG. 3:

(Color online) Energy landscapes for (a) negative and (b) positive current densities of absolute value 5 × 10⁷ A/cm² for different degrees of asymmetries δ. We define θ as the angle between the magnetization of free and pinned layers such that parallel and anti-parallel configuration, highlighted as P and AP, are found in θ = 0 and θ = π, respectively. We considered K_eff(0) = 29.5 kJ/m³ and ξ = 20 kJ/(V · m³). The solid black curve shows contribution of perpendicular anisotropy only, whereas the other curves show the total energy landscape resulting from the sum of IEC and VCMA contributions.